This invention pertains to derailleur shift mechanisms for chain-driven bicycles having one or more front chainrings secured to a crank driven by pedals, and wherein several rear sprockets are coaxially mounted on the rear wheel. Such derailleur mechanisms guide the chain onto one or another of the rear sprockets, keep tension on the chain to prevent its derailment, and take up and play out chain as required.
Current derailleur mechanisms have swinging arm tensioning and take-up devices and pantograph linkages to allow lateral adjustment of a guide pulley by the cyclist.
A pantograph support member is bolted to the bicycle frame. Two parallel pantograph arms are pivotally supported on the pantograph support member. At their other ends, the two pantograph arms in turn pivotally support a pantograph head member. A guide pulley is rotatably supported on the head member. A spring-biased swinging arm is pivotally supported also on the head member. A take-up pulley is rotatably mounted on the swinging arm. A control cable shifts the head member laterally, causing the guide pulley to line up with one or another of the rear sprockets, thus causing the chain to engage the desired rear sprocket.
Over a period of time, the swinging arm take-up mechanism has reached the limit of its capacity. Larger sprocket differences have required the take-up to be able to take up and play out more chain. Swinging arms have become longer, heavier, and perforce more subject to inertia. When a bicycle hits a bump, the weight of the swinging arm may cause tension in the chain to be momentarily relaxed, occasionally resulting in chain deraillment.
Longer swinging arms have reduced ground clearance. Current swinging arms preclude the use of wide-ratio gearing with 24" wheels to some extent; they preclude the use of such gearing almost entirely with 20" wheels.
More front chainrings and more rear sprockets have increased the misalignment of the chain, resulting in misalignment of the take-up pulley, which remains in the same lateral plane as the guide pulley.
Existing derailleur mechanism are complex mechanically, difficult to service, and make wheel removal unduly difficult. If a torsion spring of the swinging arm breaks, more often than not, the entire unit is replaced. Such derailleur mechanism are too complex to be used to any extent anywhere but in Europe, Japan, and the United States. Other parts of the world use internally geared 3 and 5 speed hubs, which, while being complicated in their own right, at least are very reliable.
What's needed is a derailleur mechanism which will:
1. take up and play out large amounts of chain. PA1 2. accomodate much chain misalignment. PA1 3. provide uniform chain tension and prevent chain deraillment. PA1 4. Provide increased ground clearance. PA1 5. Be simple to make, mount, service, and understand.
And that's what the derailleur mechanism of the current invention provides.